Conduction Band Transitions Research Articles

Photoelectric Studies as the Key to Understanding the Nonradiative Processes in Chromium Activated NIR Materials.

In this study, we synthesized a series of Ga1.98-xInxO3:0.02Cr3+ materials with varying x values from 0.0 to 1.0, focusing on their broadband near-infrared emission and photoelectric properties. Interestingly, photocurrent excitation spectra exhibited behavior consistent with the absorption spectra, indicating the promotion of carriers into the band structure by the 4T1, and 4T2 states of Cr3+ ions. This association suggests that photocurrent in this material is influenced not only by valence to conduction band transitions but also by transitions involving Cr3+ dopants. Our investigation of luminescence quenching mechanisms revealed that nonradiative processes were not directly linked to thermally induced relaxation from the excited state 4T2 to the ground state 4A2, as usually suggested in the literature for this type of material. Instead, we linked it to the thermal ionization of Cr3+ ions. Unexpectedly, this process is unrelated to the transfer of electrons from Cr3+ impurities to the conduction band but is associated with the formation of holes in the valence band. This study provided novel evidence of luminescence quenching via the hole-type thermal quenching process in Cr3+-doped oxides, suggesting potential applicability to other transition metal ions and host materials. Finally, we demonstrated the dual-purpose nature of Ga1.98-xInxO3:0.02Cr3+ as a practical emitter for NIR-pc-LEDs and effective photocurrent for UV detectors. This versatility underscores these materials' practicality and broad application potential in optoelectronic devices designed for near-infrared and ultraviolet applications.

Spectral current density and responsivity scaling for Fourier transform photocurrent spectroscopy.

We propose and experimentally verify a methodology to scale arbitrary units to photocurrent spectral density (A/eV) in Fourier transform Photocurrent (FTPC) spectroscopy. We also propose the FTPC scaling to responsivity (A/W), provided a narrow-band optical power measurement is available. The methodology is based on an interferogram waveform consisting of a constant background and interference contribution. We also formulate conditions that have to be met for correct scaling. We experimentally demonstrate the technique on a calibrated InGaAs diode and weak responsivity, long response time SiC interdigital detector. We identify a series of impurity-band and interband transitions in the SiC detector and slow mid-gap to conduction band transitions.

Green-Lighting the Sub-Band Gap Excitation in Two-Dimensional Zinc Oxide.

Solar spectrum and sensitivity of human eyes peak at green wavelength range of visible light, and the materials that can respond to a larger part of the visible spectrum are highly sought after. Two-dimensional graphene-like zinc oxide (gZnO) is a wide band gap semiconductor, but photogeneration of electron-hole pairs in it at visible wavelengths has not been achieved so far. Here, the sub-band gap excitation in 2D zinc oxide layers covered with gold nanoparticles is reported. The sub-band gap excitation and corresponding emission are correlated with oxygen interstitials introduced by AuNP deposition in the gZnO lattice. Attachment of AuNPs on gZnO also leads to increased electron availability at oxygen sites of the gZnO lattice, which translates into greater electron availability for sub-band gap excitation. The plasmonically enhanced trap level to conduction band transition constitutes sub-band gap excitation and manifests itself in local surface potential measurements carried out using a Kelvin probe force microscope.

Modulation of electrical and thermal transports through lattice distortion in BaTi1–x Nb x O3 solid solutions

The electron and heat transports in solids are through the movement of carrier electrons and quantized lattice vibrations (phonons), which are sensitive to the lattice distortion and ionized impurities, and are essential aspects for the development of novel thermoelectric materials. In this study, we systematically investigated the modulations of electrical and thermal conductivities of BaTi1–x Nb x O3 solid solution (BTNO, 0 ≤ x ≤ 1) epitaxial films. At room temperature, BaTiO3 belongs to tetragonal perovskite and exhibits electron conduction through doubly degenerated Ti 3d-t2g orbitals upon doping, while BaNbO3 belongs to cubic perovskite and exhibits metallic electron conduction through partially filled triply degenerate Nb 4d-t2g orbitals. By controlling the Ti/Nb ratio, we found a dual modulation effect on both the lattice structures and conduction band, which affects the electrical and thermal conductivities. Similar to the SrTi1–x Nb x O3 solid solution (STNO, 0 ≤ x ≤ 1) system, a phase transition was detected at x ∼ 0.5, at which both the electron and heat transports exhibit abrupt changes. Unlike the transition in STNO, which was attributed to a polaronic phase transition, the transition in BTNO was due to contributions from both the lattice distortion and polaron effect. By controlling the lattice distortion, conduction band, and polaronic phase transitions, the electrical and thermal conductivity of BTNO epitaxial films are modulated within a much greater range than those of the STNO epitaxial films. Due to the double contribution of electron carriers and phonon to thermal conductivity (κ), the maximum κ modulation ratio of BTNO epitaxial films was ∼6.9. Our research provides an effective route to design electrical/thermal management materials.

Experimental and Theoretical Investigation into the Polaron Structure of K-Doped Polyfluorene Films

The evolution of the electronic structure and optical transition upon n-doping of poly(9,9-dioctylfluorene) (PFO) films is elucidated with photoelectron spectroscopy, optical absorption, density functional theory (DFT), and time-dependent DFT (TD-DFT) calculations. Optical absorption measurements extending into near infrared show two low-energy absorption features at low doping ratios and an additional peak at a higher energy of ∼2.2 eV that disappears with increasing doping ratios. A gap state (i.e., polaronic state) close to the Fermi level and a significantly destabilized highest valence band appear in the experimentally measured ultraviolet photoelectron spectra. These experimental results are interpreted by the TD-DFT calculations, which show that the lower energy peaks originate from the excitation from polaronic states to the conduction band, while the higher energy peak mainly originates from the destabilized valence band to conduction band transitions and only appears at low doping ratios (cred ≤ 50%, 0.5 potassium atom per fluorene monomer). The DFT calculations further indicate that polaron pairs rather than bipolarons are preferentially formed at high doping ratios. Comparing the results of doped glassy and β-phase films, we find that the ordered segments in the β-phase film disappear due to the dopant (potassium) insertion, resulting in a similar polaronic structure.

Luminescence in Manganese (II)-Doped SrZn2S2O Crystals From Multiple Energy Conversion.

Under the excitation of ultraviolet, X-ray, and mechanical stress, intense orange luminescence (Mn2+, 4T1 → 6A1) can be generated in Mn2+-doped SrZn2S2O crystal in orthorhombic space group of Pmn21. Herein, the multiple energy conversion in SrZn2S2O:Mn2+, that is, photoluminescence (PL), X-ray-induced luminescence, and mechanoluminescence, is investigated. Insight in luminescence mechanisms is gained by evaluating the Mn2+ concentration effects. Under the excitation of metal-to-ligand charge-transfer transition, the most intense PL is obtained. X-ray-induced luminescence shows similar features with PL excited by band edge UV absorption due to the same valence band to conduction band transition nature. Benefiting much from trap levels introduced by Mn2+ impurities, the quenching behavior mechanoluminescence is more like the directly excited PL from Mn2+ d-d transitions. Interestingly, this concentration preference leads to varying degrees of spectral redshift in each mode luminescence. Further, SrZn2S2O:Mn2+ exhibits a good linear response to the excitation power, which makes it potential candidates for applications in X-ray radiation detection and mechanical stress sensing.

Floquet spectrum and electronic transitions of tilted anisotropic Dirac materials under electromagnetic radiation: Monodromy matrix approach

We analyze the quasienergy-spectrum and the valence to conduction-band transition probabilities of a tilted anisotropic Dirac material subject to linearly and circularly polarized electromagnetic fields. The quasienergy-spectrum is numerically calculated from the monodromy matrix of the Schr\"odinger equation via the Floquet theorem for arbitrarily intense electromagnetic fields. To asses the valence to conduction-band transition times we deduced a Rabi-like formula in the rotating wave approximation. In the strong-field regime the spectrum as a function of the momentum components divides into two very distinctive regions. In the first, located around the Dirac point, the quasi-spectrum is significantly distorted by the field as the electronic parameters are renormalized by electronic-dressing. In the second, all the characteristics of the free carrier spectrum are retained. Linearly polarized light anisotropically deforms the spectrum according to the field polarization direction. Dirac-like points form around the original Dirac point. The quasi spectrum of circularly polarized light, instead, exhibits a gap formation in the Dirac point and has elliptical symmetry. We show that, in contrast to the single-photon resonant transitions that characterize the weak-field regime, the strong-field regime is dominated by multiphoton resonances.

Width Dependent Two-Photon Absorption in Monolayer Black Phosphorus Nanoribbons

Black phosphorus nanoribbons (BPNs) might offer alternatives to narrow-gap compound semiconductors for tunable optoelectronics in infrared region. In this work we present a quantum perturbation theory on two-photon absorption (TPA) in monolayer armchair-edged black phosphorus nanoribbons (acBPNs) employing the reduced two-band model within the long-wavelength BP Hamiltonian. The matrix elements for one-photon transition have been derived and the TPA spectrum associate with intra conduction band transition and inter band transition have been drawn. The calculations predict that the TPA coefficient in acBPNs is in the magnitude of 10−6 m/W in visible region, which is 4 orders higher than the conventional semiconductor quantum dots. And in infrared region, there is a giant TPA coefficient, which is mainly contributed from intra band transitions and can reach up to10−1 m/W. The TPA peaks can be tuned both by the width of BPNs and the electron relaxation energy.

Weak Exciton–Phonon Coupling in CdSe Nanoplatelets from Quantitative Resonance Raman Intensity Analysis

Resonance Raman spectra, cross sections, and depolarization ratios have been measured for 4.5 monolayer thick CdSe nanoplatelets dispersed in chloroform. Five excitation wavelengths between 514.5 and 476.5 nm were employed. The resonance Raman spectra are dominated by the longitudinal optical (LO) phonon near 201 cm–1 and its overtone, as in CdSe quantum dots. The absolute scattering intensity is much higher for excitation on resonance with the sharp, lowest-energy heavy-hole to conduction band transition than with higher energy transitions, decreasing by about a factor of 30 between 514.5 and 496.5 nm excitation. The LO phonon overtone is weak directly on resonance with the heavy-hole transition but much stronger at higher excitation energies, a result that is reproduced by simulations of the spectra using standard resonance Raman intensity theory. The absolute Raman cross sections imply a Huang–Rhys parameter for the LO phonon of about 0.08 on resonance with the lowest heavy-hole transition. This is a f...

Electronic and optical properties of single-layer MoS2

The electronic structures of a MoS2 monolayer are investigated with the all-electron first principle calculations based on the density functional theory (DFT) and the spin-orbital couplings (SOCs). Our results show that the monolayer MoS2 is a direct band gap semiconductor with a band gap of 1.8 eV. The SOCs and d-electrons in Mo play a very significant role in deciding its electronic and optical properties. Moreover, electronic elementary excitations are studied theoretically within the diagrammatic self-consistent field theory. Under random phase approximation, it shows that two branches of plasmon modes can be achieved via the conduction-band transitions due to the SOCs, which are different from the plasmons in a two-dimensional electron gas and graphene owing to the quasi-linear energy dispersion in single-layer MoS2. Moreover, the strong optical absorption up to 105 cm-1 and two optical absorption edges I and II can be observed. This study is relevant to the applications of monolayer MoS2 as an advanced photoelectronic device.

Determination of the energy structure of recombination centers in heavily doped AlxGa1-xN:Si epitaxial layers with x > 0.5

The photoluminescence properties of the intensive defect-related emission in heavily doped AlxGal-xN:Si layers with x > 0.5 have been investigated by photoluminescence (PL) spectroscopy. The PL band in AlN was attributed to donor-acceptor (DA) transitions. At the lowest Al content, the impurity band merges with the conduction band and DA transitions are replaced by electron-acceptor transitions involving the same acceptor. The energy structure of recombination centers was obtained using the model of configuration coordinates for Al0.67Ga0.33N.

Plasmon Modulation Spectroscopy of Noble Metals to Reveal the Distribution of the Fermi Surface Electrons in the Conduction Band

To directly access the dynamics of electron distribution near the Fermi-surface after plasmon excitation, pump-probe spectroscopy was performed by pumping plasmons on noble-metal films and probing the interband transition. Spectral change in the interband transitions is sensitive to the electron distribution near the Fermi-surface, because it involves the d valence-band to the conduction band transitions and should reflect the k-space distribution dynamics of electrons. For the continuous-wave pump and probe experiment, the plasmon modulation spectra are found to differ from both the current modulation and temperature difference spectra, possibly reflecting signatures of the plasmon wave function. For the femtosecond-pulse pump and probe experiment, the transient spectra agree well with the known spectra upon the excitation of the respective electrons resulting from plasmon relaxation, probably because the lifetime of plasmons is shorter than the pulse duration.

Analysis of photon emission induced by light and heavy ions in time-of-flight medium energy ion scattering

Abstract We present a systematic analysis of the photon emission observed due to impact of pulsed keV ion beams in time-of-flight medium energy ion scattering (ToF-MEIS) experiments. Hereby, hydrogen, helium and neon ions served as projectiles and thin gold and titanium nitride films on different substrates were employed as target materials. The present experimental evidence indicates that a significant fraction of the photons has energies of around 10 eV, i.e. on the order of typical valence and conduction band transitions in solids. Furthermore, the scaling properties of the photon emission with respect to several experimental parameters were studied. A dependence of the photon yield on the projectile velocity was observed in all experiments. The photon yield exhibits a dependence on the film thickness and the scattering angle, which can be explained by photon production along the path of the incident ion through the material. Additionally, a strong dependence on the projectile type was found with the photon emission being higher for heavier projectiles. This difference is larger than the respective difference in electronic stopping cross section. The photon yield shows a strong material dependence, and according to a comparison of SiO2 and Si seems to be subject to matrix effects.

Partially polycrystalline GaN1−xAsx alloys grown on GaAs in the middle composition range achieving a smaller band gap

GaN1−xAsx alloys have been successfully grown on (100) GaAs substrates over a wide composition range (0.15 < x < 0.98) by plasma-assisted molecular beam epitaxy. In the middle composition range, the weak and broad (111) diffraction peaks are observed in the X-ray diffraction patterns. These diffraction peaks most likely come from small crystalline grains within the amorphous matrix and are unlike the entirely amorphous GaNAs alloys grown on sapphire and Pyrex glass. A transmission electron microscopy micrograph of the GaN0.50As0.50 alloy also shows a weak periodic structure consisting of small polycrystalline grains. To study the band gap and the As-affected spin–orbit band to conduction-band minimum transition, photomodulated reflectance is utilized. The band gap energies range from 0.78 to 2.15 eV (3.4 eV for end-point compounds GaN). Finally, the original and modified band anticrossing (BAC) models for GaNAs alloys were thoroughly verified over the entire composition range. Remarkably, the band gap energies of the partially polycrystalline GaNAs alloys agree well with those obtained using the original BAC model in the middle composition range because the model has been developed for crystalline materials. These results improve the growth of highly mismatched GaNAs alloys with different substrates and should expedite studies of high-efficiency multijunction solar cells fabricated using such a single ternary alloy system.

RGB emission of Mn2 + doped zinc phosphate glass

We report the photoluminescence (PL) of Mn2+ doped zinc phosphate glass as melted. Raman, X-Ray diffraction (XRD), Energy-Dispersive X-ray Spectroscopy (EDS) were used to get a better characterization. During the synthesis process, the Mn2+ ions, tetrahedrally coordinated (IVMn2+) in the glass, partially precipitate on octahedral sites (VIMn2+) that gives place to the simultaneous occurrence of green and red luminescence, due to the spin-forbidden 4T1(G)→6A1(S) and 4T1g(G)→6A1g(S) transitions in IVMn2+ and VIMn2+ respectively. The ratio IVMn2+/VIMn2+ can be controlled by the increase of %Mn in the glass and, hence, the ratio between green and red emissions can be controlled. The change of Mn2+ ions coordination form 4 to 6 is seen by Electron Paramagnetic Resonance (EPR) and lifetime measurements. PL results indicate the precipitation of crystalline ZnO phases with relatively small particle size that produces its characteristic blue emission due to conduction band to valence band exciton transitions. The transition 6A1(S)→4E(D) of Mn2+ centered at 350nm can produce a blue and red dual luminescence by the overlap with the valence band to conduction band transition of ZnO, while the transition 6A1(S)→4A1(G) of Mn2+ centered at 409nm produces the green and red dual luminescence that is dependent on %Mn in the glass.

Optical properties of intermixed vertical cavity surface emitting lasers: a theoretical model

The effect of intermixing on some important optical properties of vertical cavity surface emitting lasers (VCSELs) has been successfully modeled and analyzed. A combined superposition of the error function solution to the diffusion equation for a hypothetical AlxGa1−xAs/GaAs VCSEL structure has been applied to simulate the intermixing process during growth or post-growth thermal annealing using a depth-independent and/or a depth-dependent interdiffusion coefficient. The simulations of the VCSEL reflectivity and phase shift revealed no appreciable difference between the two assumptions. It has been shown that for diffusion lengths of about 20 nm the overall VCSEL reflectivity and phase shift does not change significantly, however, the VCSEL bandwidth was shown to decrease at relatively small diffusion length. The width of the transmission “dip”, which is the VCSEL emission wavelength was also shown to increase with increasing diffusion length indicating an appreciable intermixing of the active region and hence different conduction and valence band transitions due to modifications in the quantum wells and barriers shapes and widths. The presented model may provide a simple yet versatile technique for the analysis of interdiffusion during VCSEL growth or post-growth thermal annealing for device efficiency and optimization.

Theoretical analysis of direct transition in SiGeSn/GeSn strain balanced QWIP

This work presents a theoretical analysis of direct transition in strain balanced SiGeSn/GeSn quantum well infrared photodetector. Eigen energies for Г valley conduction band, heavy hole band and light hole band are obtained from the self consistent solution of coupled Schrodinger and Poisson equations by finite difference method. Absorption spectra for direct transition of heavy hole and light hole band to Г valley are calculated after evaluating Eigen energies and wave functions. Significant absorption in infrared region is obtained for heavy hole band–Г valley conduction band transition. A significant shift in absorption peak towards longer wavelengths is observed in presence of electric field considering excitonic effect.

Size-dependent two-photon absorption in circular graphene quantum dots.

We investigate theoretically the size-dependence of two-photon absorption (TPA) for circular graphene quantum dots (GQDs) on the basis of electronic energy states obtained by solving the Dirac-Weyl equation analytically under infinite-mass boundary condition. The analytical expressions for TPA coefficient are derived with an arbitrary size-distribution and the transition selection rules are obtained. Results reveal that the intraband transitions in conduction band and valence band contribute much more to TPA than interband transitions. The energy spectrum and TPA peaks are tuned by the size of GQDs.

Enhanced Luminescence with Fast Nanosecond Lifetime in In2S3:Tb3+ Nanophosphors

Luminescent inorganic nanoparticles have attracted large amounts of attention because of their enhanced luminescence efficiency and shortening of radiative lifetime, particularly rare earth ion doped nanophosphors that possess excellent luminescent properties as a result of their inner shell electronic transitions between the 4f–4f energy levels. Herein, we have investigated the effect of Tb3+ content on the absorption, photoluminescence, and luminescence decay curves. The PL spectra give two emission processes, which are assigned to the intrinsic emission and the defect-related emission, respectively. Excess terbium content leads to reducing intensity of photoemission as a result of concentration quenching. The fitted lifetime values are approximately 1 ns and get shortened with the increase of Tb3+ content, due to the increased nonradiative transition rate. UV absorption corroborates valence–conduction band transition and strong quantum size effect. The energy band gap value for Tb3+-doped In2S3 is high...

Significance of host׳s intrinsic absorption band tailing on Ce3+ luminescence quantum yield in borate glass

In glass, the valence band to conduction band transition exhibits an extended band tailing. This band tailing is an intrinsic property of the glass and is consists of the exponential Urbach tail and the defects induced weak absorption tail. The extended nature of the band tailing incorporates finite absorption of the electromagnetic radiation in the band gap region. In this work, the contribution of such host׳s intrinsic band tail absorption on Ce3+ luminescence quantum yield has been investigated in the borate glass host. The absorption spectra revealed that the host’s band tailing effectively overlaps the Ce3+ excitation bands in the borate glass that causes an unwanted absorption loss during the Ce3+ excitation. This gives rise to a significant reduction in the measured external quantum efficiency of Ce3+ luminescence. A comparison of standard quantum yield (~42%) and host corrected quantum yield (~62%) suggests that the host’s intrinsic absorption gives rise to about 20% reduction in Ce3+ luminescence quantum yield in the studied borate glass.